This invention relates to a measuring instrument, and in particular to a flowmeter, whose measuring operation, i.e. the quantitative determination of a given variable, serves to quantify an output value as a function of and with reference to a known input value. The invention further pertains to a method for testing the functional performance, i.e. the measuring operation, of a measuring instrument, especially that of a flowmeter whose measuring operation i.e. its determination of a measured value, consists in the quantification of an output variable as a function of a known input value.
In this case, the term input value refers to a value generally generated within the measuring instrument and employed in the determination of a measured variable on the basis of the interactive relationship between the input value and the measured variable. It is this interactive relationship which generates the measured variable, i.e. the output value. One example intended to illustrate the aforementioned interrelationship between measured variable, input value and output value is the determination of the flow rate of a fluid moving through a pipeline, using a magnetoinductive flowmeter. In the case of such a magnetoinductive flowmeter, a magnetic field is generated providing the input value. The latter interacts with the measured variable, that being the flow rate of the moving fluid, inducing in the flowing fluid a voltage that constitutes the output value.
It is an established practice to subject measuring instruments to a so-called self-test before or during the measuring operation. A self-test of that nature may serve to determine whether all the components of the measuring instrument are fully operational and also whether the measuring operation itself is properly executed. When the result of such a self-test is positive, meaning that there is no malfunction within the measuring instrument or in the measuring operation, one is assured that, within established tolerances of course, the measured values obtained with the measuring instrument reflect actual, true values.
For example, as the data are entered, this type of self-test includes plausibility verification of the setpoint values. Entries outside the permissible range, rather than being accepted, are rejected. Independent of this verification of the setpoint values at the time of the data entry, the data which are computationally derived from the setpoint values and which directly control the functional operation of the measuring instrument, are once more examined for their permissible limits. This examination takes place prior to the measuring operation, triggering in the event of a setpoint-related error a message such as xe2x80x9cFatal Errorxe2x80x9d and preventing the measuring operation from being executed. For example, it is possible to test individual components and modules of the measuring instrument for their functional performance for instance by impedance measurements or by measuring their ohmic resistance or their conductivity.
These prior-art self-testing capabilities of measuring instruments notwithstanding, there remains a need for additional self-testing procedures which allow an even more accurate determination of whether there is a malfunction in the measuring operation or in the measuring instrument itself. In particular, there may be cases where individual components of the measuring instrument deviate only slightly from their setpoint values, causing a self-test to suggest completely proper functionality of each such component, and yet, as these components interact, the resulting measuring operation may be flawed and may not necessarily ensure the specified accuracy of the measured values.
It is therefore the objective of this invention to provide a measuring instrument, and a method for testing the functional performance of such a measuring instrument, which during the ongoing measuring operation of the measuring instrument permit in simple fashion a very precise and accurate detection of any malfunctions.
The measuring instrument according to the invention as outlined above and designed to achieve this objective is characterized in that the input value is variable, that the output value which corresponds to any given input value can be measured, and that on the basis of any threshold-exceeding deviation of the output value, measured in relation to different input values, from the output value that is expected as a function of the relationship between it and the input value, a malfunction in the measuring operation is detectable. Thus, the invention is based on the use of the essentially known functional relationship between the input value and the output value, fundamentally underlying the measuring operation, for testing that measuring operation. In the simplest case, this interrelationship between the input value and the output value is a linear function, permitting the detection of a malfunction in the measuring operation of the measuring instrument simply by virtue of a deviation of the output values, derived from mutually different input values, by more than a certain threshold value from a linear pattern.
The invention provides for the input value to be varied. This means that the measuring operation employs at least two time-differentiated input values. In a preferred implementation of the invention, the input value can be varied in terms of its amplitude on a time and preferably cyclical basis. However, the invention is not limited to a time-based variation of the amplitude of the input value but is similarly applicable in the form of a variation of other suitable parameters of the input value.
In comparing the actually measured output value with the output value expected based on the corresponding input value, any deviation can be separately used for test purposes. However, in a preferred implementational version of the invention, the deviation can be averaged over several values, preferably at least 100. This prevents individual xe2x80x9cstray componentsxe2x80x9d of the measured output value from erroneously suggesting a malfunction in the measuring operation. As an alternative it is also possible to subject the measured deviation to low-pass filtering. In another preferred implementation of the invention, more complicated measuring conditions due for instance to an augmented standard deviation relative to the measured actual deviation are addressed by an increase in the number of the values over which the deviation can be averaged. This latter approach is equally useful in the case of strongly fluctuating measured values for avoiding the erroneous indication of a malfunction in the measuring operation.
The threshold value which, when exceeded, allows the detection of a measuring malfunction of the measuring instrument, can be established in different ways, one example being a constant absolute value. However, in a preferred implementation of this invention, the threshold value is established as a percentage of the rated measuring range, i.e. of full scale, for the output value. In this fashion it is possible, by one single definition of the threshold value as a percentage of full scale for the output value, to arrive at a setting for the measuring instrument which can be used for all selectable measuring ranges of the measuring instrument. In this context, according to a particularly preferred embodiment of the invention, the percentage of full scale for the output value is 1% or less.
The detection of a measuring malfunction of the measuring instrument can prompt different courses of action, especially a stoppage of the measuring operation of the measuring instrument, since obviously no reliable measuring results can be obtained any longer. However, in a preferred implementation of this invention the detection of a measuring malfunction can trigger an optical or acoustic alarm signal.
This invention is fundamentally suitable for any measuring instrument which must meet the above-mentioned requirements, involving a measuring operation in which an output value that is a function of an input value is measured for the determination of an object value to be measured. The method according to this invention is particularly useful, however, when applied in magnetoinductive flowmeters for measuring the flow rate of a conductive fluid through a pipeline, where the input value is a field current fed into the field coils of the magnetoinductive flowmeter, the output value is a voltage that is induced in the conductive fluid and retrievable at the test electrodes, and a malfunction of the measuring operation of the magnetoinductive flowmeter can be detected when the deviation of the induced voltage measured at different field currents from a linear voltage curve is in excess of the established threshold value.
The method of this invention, based on the above-described methodology applied in the testing of the measuring operation of a measuring instrument and designed to achieve the stated objective, is characterized in that the input value is varied, the output value corresponding to a given input value is measured and the threshold-exceeding deviation of the measured output values, derived from different input values, from the output value expected as a function of the interrelation between the latter and the input value, is used to determine a malfunction of the measuring operation.
Preferred versions of the method according to this invention for testing the measuring operation of a measuring instrument are adapted in analogous fashion to the above-described implementation versions of the measuring instrument of this invention.